The present invention relates to audio and data signal processing in a television waveform, and more particularly to the use of a television waveform to carry a plurality of separate digital audio/data channels.
There are several known systems for transmitting scrambled digital audio information in nonvideo portions of a video waveform. Two such systems are disclosed in U.S. Pat. No. 4,563,702 entitled "Video Signal Scrambling and Descrambling Systems" to Heller, et al and U.S. Pat. No. 4,613,901 entitled "Signal Encryption and Distribution System for Controlling Scrambling and Selective Remote Descrambling of Television Signals" to Gilhousen, et al, both incorporated herein by reference. These systems provide for digital stereo audio transmission in the horizontal blanking interval of a National Television Standards Committee ("NTSC") television signal.
The systems disclosed in the above referenced patents are primarily concerned with the delivery of television programming which contains both audio and video components. In another known system described in U.S. Pat. No. 4,787,085 entitled "Digital Signal Transmitting System" to Suto, et al, digital signals that are not limited to audio and video components of a television signal are time division multiplexed and modulated to form a signal having a bandwidth corresponding to one channel portion of a television broadcast signal. A receiver extracts the digital data corresponding to a desired signal and supplies it to an appropriate appliance such as a computer, stereo set, or facsimile machine that converts the data into a useful form for a user. In the Suto, et al system, all signals are transmitted in a digital format, requiring a separate receiver that is not compatible with existing cable television converters or satellite television receivers.
In order to increase revenues, subscription cable and satellite television system operators are interested in adding new services to their product offerings. With the advent of high quality digital audio, popularized by the development of compact disc ("CD") players, a market for CD quality audio delivered by subscription television systems has become apparent. In order to reduce costs for the system operators, however, compatibility with existing equipment both at the headend and at the subscriber locations is desired. It would therefore be advantageous to provide a system wherein multiple stereo audio channels having CD quality can be provided in lieu of video information in a standard NTSC television waveform. One of a number of such audio channels might then be selectively decoded based upon a subscriber's input to the decoding device. In a decoder that can accommodate both television signals containing digitized audio in nonvideo portions of the waveform, and waveforms in which video information has been replaced by digitized audio information, a cost advantage can be realized by using common audio detection, demultiplexing and processing circuitry.
The present invention provides a system having the advantages described above. While several prior art multiplexed schemes for multichannel audio only service may exist, the present invention provides the specific advantage of utilizing multiplexed audio data intervals that are matched in periodicity and fit within a time period equal to the horizontal blanking interval ("HBI") of a television waveform.
One object of the present invention is to provide a system for transmission of a plurality of audio and auxiliary data channels in a video waveform by replacing the video portion thereof with a group of audio channels (which may have auxiliary data appended thereto) while also providing digital audio data in nonvideo portions of the waveform. It is a further object of the present invention to enable circuitry that is normally used to receive and decode audio signals present in nonvideo portions of a television waveform to provide decoding of audio and auxiliary data signals that have replaced the video portion of the waveform. It is a still further object of the present invention to provide for the implementation of a scrambled multiple audio service subscriber system using the same key distribution system that is used to secure and provide entitlements for conventional subscription video programming.